Lipids: Classification and Properties
Saponifiable Lipids
Saponifiable lipids contain fatty acids, which are monocarboxylic acids that may or may not be unsaturated. When subjected to alkaline hydrolysis, they undergo saponification, forming soaps (salts of fatty acids). Examples include glycerides, waxes, phospholipids, and sphingolipids.
Unsaponifiable Lipids
Unsaponifiable lipids do not contain fatty acids and therefore do not undergo saponification reactions. This group includes terpenes, steroids, and prostaglandins.
Fatty Acids
These are monocarboxylic organic acids with an even number of carbon atoms. They can be free or part of a lipid molecule.
Saturated Fatty Acids
Saturated fatty acids lack double bonds and are usually solid at room temperature. Palmitic and stearic acids are the most abundant.
Unsaturated Fatty Acids
Unsaturated fatty acids have one or more carbon double bonds in their chain and are liquid at room temperature. Oleic acid (monounsaturated) and linoleic, linolenic, and arachidonic acids (polyunsaturated) are important examples.
Physicochemical Properties
Fatty acids are amphipathic, possessing a polar hydrophilic carboxyl group (-COOH) and a nonpolar hydrophobic carbon chain. They react with alcohols to form esters and release water. They are hydrolyzed in the presence of alkali, forming salts. The degree of unsaturation and chain length determine the melting point, which increases with chain length due to increased Van der Waals interactions. Double bonds introduce bends in the molecules, reducing interactions and lowering the melting point.
Glycerides (Fats)
Glycerides are compounds consisting of glycerol esterified with one, two, or three fatty acids. Triglycerides are the most abundant, with three fatty acids that can be the same or different. Fats are nonpolar and practically insoluble in water because the polar hydroxyl groups of glycerol are linked to the carboxyl groups of fatty acids through ester bonds.
Fats of Vegetable Origin
These fats mainly contain unsaturated fatty acids, resulting in low melting points and a liquid state at room temperature.
Fats of Animal Origin
These fats mostly contain saturated fatty acids, leading to higher melting points and a solid state at room temperature. Fats serve as the primary energy reserve in both animals and plants, accumulating in vacuoles (plants) and adipocytes (mammals). They also provide thermal insulation.
Waxes
Waxes are esters of a long-chain fatty acid and a long-chain monoalcohol.
Phospholipids
Also known as phosphoglycerides, phospholipids are major components of biological membranes. They consist of glycerol esterified at carbon 3 with a phosphate group and at carbons 1 and 2 with fatty acids. Typically, the fatty acid at C1 is saturated, while the one at C2 is unsaturated. The phosphate group is linked to a polar substituent (aminoalcohol or polyalcohol). Phosphatidic acid is a simple phospholipid with stearic acid at C1, oleic acid at C2, and a phosphate group. Phospholipids are amphipathic, with a hydrophilic polar region (phosphate group and polar substituents) and a hydrophobic nonpolar region (fatty acids). This amphipathic nature allows them to form bilayers and micelles in aqueous environments.
Phospholipids in Biological Membranes
In aqueous environments, phospholipids arrange themselves to form structures where hydrophilic groups interact with water molecules through hydrogen bonds, while hydrophobic groups interact with each other through Van der Waals forces. This leads to the formation of bilayers (hydrophobic chains inside, polar heads facing the aqueous medium) and micelles (spherical structures with polar heads on the surface and hydrophobic chains inside). Liposomes, composed of phospholipid bilayers enclosing an aqueous compartment, can be created under laboratory conditions. The amphipathic nature of phospholipids is crucial for the formation of biological membranes in both prokaryotic and eukaryotic cells. Besides phospholipids, these membranes also contain proteins and other lipids like cholesterol and sphingolipids.
Sphingolipids
Sphingolipids are structurally and functionally similar to phospholipids, being amphipathic and forming bilayers in aqueous media. They are present in all eukaryotic cell membranes but are particularly abundant in nervous system tissues. They consist of an aminoalcohol (sphingosine or its derivatives), a fatty acid, and a polar group. Sphingosine binds to a fatty acid at its amino group, forming ceramide.
Sphingomyelins
Sphingomyelins are the only sphingolipids containing a phosphate group. They are found in animal cell membranes, especially in the myelin sheath surrounding nerve fibers.